56 kbps modems are very soon to be standardized in accordance with the V.90 modem recommendation. V.90 modems employ pulse code modulation (PCM) techniques and take advantage of the digital nature of the public switched telephone network (PSTN) to obtain a higher data rate in the downstream direction, i.e., from a server modem to a client modem. FIG. 1 depicts a conceptual diagram of a typical 56 kbps communication path using current PCM modem technology. A central site, such as an internet service provider (ISP) 100, is digitally connected to a telephone network 130 through a transmitter 110 and a receiver 120 resident at an ISP modem 105. The network 130 is connected to a local loop 150 through a central office line card 140. The line card typically has a PCM codec implemented therein. The local loop 150 is connected to the user's personal computer (PC) 170 at the user's site through the user's modem 160. As can be appreciated by those skilled in the art, the connection between the ISP modem transmitter 110 to the telephone network 130 is a digital connection that supports a typical data rate of about 64 kbps. Since the parameters of the telephone network 130 and line card 140 are dictated and set by the operating specifications of the network (and particularly the use of the .mu.-law or A-law signal point constellations), the central site transmitter 110 is configured to transmit the digital data in a particular way to fully exploit its digital connection to the network.
FIG. 2 illustrates a portion of a digital server modem 200, which includes a transmitter 202 and a receiver 204. As with many practical data communication systems, near-end echo (represented by an echo path 206) associated with a transmit signal may be present in a signal received by server modem 200. The characteristics of the near-end echo signal may be dictated by functional components in the upstream and downstream channels and/or processing performed within the telephone network. The echo signal combines with the intended receive signal and the "corrupted" receive signal is then processed by server modem 200. An echo canceler 208 is employed by server modem 200 to compensate for the near end echo. In an ideal modem system, a duplicate echo signal generated by echo canceler 208 is subtracted from the signal received by server modem 200.
In V.90 modems, the presence of digital impairments in echo path 206 complicates the echo cancellation process. These digital impairments, such as robbed bit signaling (RBS) and digital pads, are nonlinear in nature and usually memoryless; conventional echo cancellation techniques for modeling the overall echo path using a linear time invariant (LTI) filter may not provide optimal results. Instead, the echo path 206 can be effectively modeled as a nonlinear memoryless mapping, an LTI filter, and another nonlinear memoryless mapping. Furthermore, current echo cancellation techniques for V.90 server modems have several shortcomings. One proposed solution utilizes a probing signal that is transmitted by digital server modem 200 and received by the analog client modem. The client modem detects and analyzes the downstream digital impairments and sends information back to server modem 200. The information is utilized by echo canceler 208 to establish and model the characteristics of the downstream digital impairments in addition to analog aspects of echo path 206. This technique is undesirable because server modem 200 must rely upon the detection and analytical capabilities of the client modem. Performance errors caused by the client modem can adversely affect the server modem echo cancellation procedure and can adversely affect the ultimate resolution of the receive signal.
Furthermore, the current scheme for sending back the mapping information from the analog client modem to the digital server modem 200 assumes that the digital impairment source always maps one PCM level to another PCM level. This assumption may not be valid for some situations. In addition, the above proposed scheme only analyzes the downstream characteristics of echo path 206. The client modem has no way to directly determine those aspects of echo path 206 that are associated with any upstream digital impairment features of the modem system. Although server modem 200 may be configured to estimate such upstream digital impairment features in echo path 206, an adaptive and direct measurement of echo path 206 may be more robust and reliable.
Finally, the accuracy of this proposed echo cancellation technique may not be optimal. Due to elements in the downstream path over which the probing, signal must travel, the amount of noise in the probing signal may exceed desirable levels. Excessive noise levels or nonlinearities present in the downstream channel (which may not necessarily exist in echo path 206) can corrupt the determination of the downstream digital impairment characteristics and can lead to inaccurate echo cancellation.